The Catacombs in Paris consist of a network of subterranean tunnels and rooms in the former Roman-era limestone quarries. They were converted into a mass tomb at the end of the 18th C. At the time, parts of Paris were suffering from disease, which was due to contamination caused by improper burials and mass graves in church graveyards. The Catacombs of Paris now contain the bodies of roughly 6-7 million Parisians.

Catacombs, Paris
Photograph by Stephen Alvarez, National Geographic
This Month in Photo of the Day: National Geographic Magazine Features
In a sandy chamber known as the "beach," a wave rolls across a wall painted (and repainted) by cataphiles in the style of Japanese printmaker Hokusai. Such works can take hundreds of hours—the painting but also the carrying in of supplies.